Levothyroxine 113 mcg is an Appropriate Dose for Many Adults with Primary Hypothyroidism
Levothyroxine 113 mcg daily falls within the typical replacement range for adults with primary hypothyroidism, as most patients require 100-150 mcg/day (median 125 mcg/day) to achieve euthyroidism. 1
Standard Dosing Framework
The full replacement dose of levothyroxine for primary hypothyroidism is 1.6 mcg/kg/day, which translates to approximately 100-150 mcg/day for most adults. 1, 2 Specifically:
- Women typically require 75-100 mcg/day 3
- Men typically require 100-150 mcg/day 3
- The median dose across all patients is 125 mcg/day 1, 4
A dose of 113 mcg sits comfortably within this therapeutic range and represents a standard maintenance dose for many patients.
When 113 mcg Would Be Appropriate
For Younger Adults Without Cardiac Disease
For adults under 70 years without cardiac disease or multiple comorbidities, starting with the full replacement dose of 1.6 mcg/kg/day is safe and achieves euthyroidism faster without increasing cardiac risk. 1, 5 A prospective randomized trial demonstrated that full-dose initiation caused no cardiac complaints or events, even at bicycle ergometry testing. 5
For a patient weighing approximately 70 kg (154 lbs), the calculated dose would be:
- 70 kg × 1.6 mcg/kg = 112 mcg/day
This makes 113 mcg an appropriate full replacement dose for this weight range.
For Patients Already on Treatment
If a patient is already taking levothyroxine and their dose is 113 mcg, this would be appropriate if:
- TSH is within the reference range (0.5-4.5 mIU/L) 1, 2
- Free T4 is normal 6
- The patient is clinically euthyroid 1, 2
When 113 mcg Would NOT Be Appropriate
For Elderly or Cardiac Patients
For adults over 70 years or those with cardiac disease/multiple comorbidities, start with a lower dose of 25-50 mcg/day and titrate slowly every 6-8 weeks. 6, 1, 2 Starting at 113 mcg in these populations risks:
- Cardiac decompensation or exacerbation of angina 6, 7
- Atrial fibrillation, especially in elderly patients 6
- Unmasking of underlying coronary disease 6
For these patients, use 12.5 mcg increments when titrating to avoid cardiac complications. 1
If TSH Remains Elevated
If a patient taking 113 mcg has TSH >4.5 mIU/L, the dose is insufficient and should be increased by 12.5-25 mcg increments every 4-6 weeks until TSH normalizes. 1, 2
If TSH Is Suppressed
If TSH falls <0.1 mIU/L on 113 mcg, the patient is overtreated and requires immediate dose reduction by 12.5-25 mcg to prevent: 6, 1
- Atrial fibrillation and cardiac arrhythmias 6
- Osteoporosis and fractures 6
- Increased cardiovascular mortality 6
Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, highlighting the critical importance of regular monitoring. 6, 1
Monitoring Requirements
After initiating or adjusting to 113 mcg:
- Check TSH every 6-8 weeks during dose titration 6, 1, 2
- Once stable, monitor TSH every 6-12 months 6, 1
- The peak therapeutic effect may not be attained for 4-6 weeks 2
- Free T4 can help interpret ongoing abnormal TSH levels during therapy 6, 1
Special Populations
Pregnant Patients
Women with pre-existing hypothyroidism should increase their levothyroxine dose by 25-50% above pre-pregnancy requirements as soon as pregnancy is confirmed. 1, 2 If taking 113 mcg pre-pregnancy, increase to approximately 141-170 mcg/day during pregnancy, with monthly monitoring. 2
Patients Requiring TSH Suppression
For thyroid cancer patients, 113 mcg may be insufficient if TSH suppression is required. Target TSH varies by risk stratification: 6
- Low-risk patients: TSH 0.5-2 mIU/L 6
- Intermediate-to-high risk: TSH 0.1-0.5 mIU/L 6
- Structural incomplete response: TSH <0.1 mIU/L 6
Critical Pitfall to Avoid
Never start or increase levothyroxine before ruling out adrenal insufficiency in patients with suspected central hypothyroidism or hypophysitis, as this can precipitate life-threatening adrenal crisis. 6, 1 In such cases, corticosteroids must be initiated first. 6